ASHRAE Journal - August 2023 - 27

COLUMN INFECTIOUS AEROSOL CONTROL
requirements, but did lay the groundwork for their
development as a logical next step.
A New Standard
The development of Standard 241 began with the
December 6, 2022, ASHRAE board motion authorizing
development of a " comprehensive,
consensus-based, code enforceable
standard...to mitigate the risk from
respiratory pathogens " on an aggressive
six-month schedule requested by the
U.S. White House COVID-19 Response
Coordinator, Dr. Ashish Jha. The
committee of international experts formed
to undertake this challenging task fi rst
met on February 28, 2023, delivered a
public review draft on May 11 and, after
considering more than 1,000 comments,
recommended a fi nal draft for publication
on June 15, which was subsequently
approved on June 24. This unprecedented
development time of 116 days was made
possible by the extraordinary commitment of the project
committee and to its ability to build on existing ASHRAE
standards and the guidance of the ETF. Key aspects of
the new standard are summarized below.
* Standard 241 applies to occupiable space in new
buildings, existing buildings and major renovations,
including residential buildings and some portions of
health-care facilities. It sets requirements for outdoor
air system and air cleaning system design, installation,
commissioning, operation, and maintenance.
* A prerequisite for meeting Standard 241 is
that minimum indoor air quality be provided
by complying with the applicable version of
ANSI/ASHRAE Standard 62.1, ANSI/ASHRAE Standard
62.2, or ANSI/ASHRAE/ASHE Standard 170 at the time of
construction or major renovation.
* Standard 241 gives requirements that apply to
infection risk management mode (IRMM), which is the
operating mode when increased protection from
infectious aerosol exposure is needed. Public health
offi cials or other authorities may begin and end
required use of IRMM, but it could also be used at the
discretion of the building owner or occupant.
* Equivalent clean airfl ow requirements are the single
most important aspect of Standard 241. The equivalent
clean airfl ow requirement for a space is the product of
the number of occupants and the required equivalent
clean airfl ow per person (ECAi) for the space type.
Equivalent clean airfl ow is the " fl ow rate of pathogenfree
air that, if distributed uniformly within the
breathing zone, would have the same effect on infectious
aerosol concentration as the sum of actual outdoor
airfl ow, fi ltered airfl ow and inactivation of infectious
aerosols. " This provides the user of the standard great
fl exibility to determine how to achieve compliance.
* Standard 241 sets requirements for air distribution,
in mechanically ventilated, naturally ventilated and
mixed-mode buildings, including fl ow pattern and
placement of in-room air cleaners.
* The standard provides extensive requirements
for mechanical fi lters and air cleaners. These
include required testing for both performance
and safety. Mechanical fi lters must be at least
MERV-A 11 or equivalent to receive credit for
contributing to equivalent clean airfl ow requirements.
Prescriptive infectious aerosol removal effi ciency values
are provided. Other air cleaners and air disinfection
technologies must be tested by in-duct or chamber
methods described in Normative Appendix A of
Standard 241. Safety testing requires measurement of
formaldehyde, ozone and particulate matter emissions
and sets target levels.
* Standard 241 includes extensive requirements for
assessment, planning, commissioning, operation and
maintenance of infectious aerosol control systems,
which revolve around the development of a Building
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ASHRAE Journal - August 2023

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